Driers, for example, sludge driers, are typically heated indirectly with heat transfer oil, steam, air/air heat exchangers or similar systems and have a closed circulating drying air loop. The closed air loop carries the gas or air stream containing the water vapor evaporated from the material being dried to a condenser where the water vapor is condensed and removed from the air stream. The condenser cools the air stream and condenses the water vapor, for example, by direct water injection or by a cooling spiral. A portion of the air stream in the air loop of the drier has to be removed from the drier loop to prevent air leakage from the system at the seals or other leakage points. The exhaust air is removed to eliminate the various gases, which build up during drying or which are fed into the system.
This exhaust air from the drier loop, however, contains contaminants and/or unpleasant smells, particularly in the drying of sewage sludge. The exhaust air is usually cleaned thermally by being used as combustion air in the heating boiler or furnace of the drying plant. A disadvantage of this method is that the air drawn into the combustion chamber is only retained at the hottest part of the flame for an extremely short period of time which is insufficient to decontaminate the air. Thus, only a small degree of decontamination is possible with these prior systems. At times where there is little or no material being dried, the heating boiler or furnace is not heated to full capacity and has a lower flame temperature. This results in lower flue temperatures and insufficient decontamination of the exhaust air. Similar cases occur during the start-up and shutdown of the plant which can occur several times a day when the plant is run in a single or two-shift operation.
Accordingly, a continuing need exists for improving thermal cleaning of exhaust air from indirectly heated drying plants, both in normal operations and at lower heating levels, such as during start-up and shutdown sequences and interrupted or reduced drying capacity.